1. Field of the Invention
The present invention relates to an illumination optical system and an exposure apparatus having the optical system, and particularly, to correction of a distribution of an intensity of illumination in an illumination optical system in a semiconductor projection exposure apparatus. Also, the present invention relates to a method for manufacturing a semiconductor device using the illumination optical system.
2. Related Background Art
In a semiconductor exposure apparatus, a light ray emitted from a light source is incident on an optical integrator, and a multiple light source which is constituted by a plurality of light source images is formed on the rear focal plane of the optical integrator. The light ray from the multiple light source is passed through an aperture stop which is formed in the vicinity of the rear focal plane of the optical integrator and then is caused to enter a condenser optical system. The aperture stop causes the form or the size of the multiple light source to change into a desired one in accordance with a desired illumination condition.
The light ray condensed by the condenser optical system illuminates a reticle (or a mask) in an overlapping manner through a reticle blind which is disposed in the condenser optical system. A pattern is formed on the reticle, and an illumination area of the pattern is determined by the reticle blind. The light passed through the pattern of the reticle is imaged on a wafer through a projection optical system. Thus, the reticle pattern is projection-exposed (transferred) onto the wafer.
The pattern formed on the reticle is highly integrated, so that it is inevitably required to obtain a uniform distribution of the intensity of illumination on the wafer in order to correctly transfer the pattern onto the wafer. To this end, it is required to design and arrange the optical integrator and the condenser optical system in such a manner that the distribution of the intensity of illumination on the wafer should be uniform. However, even if the optical integrator and the condenser optical system are designed and arranged such that the distribution of the intensity of illumination becomes uniform, an unevenness in the intensity of illumination may be generated due to an error, or the like, in manufacturing the apparatus so that a uniform distribution of the intensity of illumination can not be obtained. Therefore, a conventional exposure apparatus is arranged such that a correction is properly performed by moving a movable lens group which constitutes a part of the condenser optical system in the direction of the optical axis so as to reduce a degree of unevenness in the distribution of illumination, thereby obtaining a uniform distribution of the intensity of illumination.
Recently, attention is given to the fact that the sizes of the plurality of light source images formed by the optical integrator are changed by changing the aperture form of an aperture stop or the like arranged on the exit side of the optical integrator so as to change a coherency .sigma. (.sigma.=the aperture stop size/ the pupil size of the projection optical system) of the illumination, or that the forms of the plurality of light source images formed by the optical integrator are changed into those of annular band or the like so as to improve the original depth of focus or degree of resolution of the projection optical system.
However, it has been clearly seen from various kinds of repeated experiments, etc., that, together with such change of the aperture form of the aperture stop or the like arranged on the exit side of the optical integrator, an unevenness of the intensity of illumination occurs on a reticle which serves as a plane to be irradiated, or a wafer which serves as a photosensitive substrate.
Accordingly, the present invention has been conceived taking the above-mentioned problem into consideration, and an object of the invention is to provide an illumination optical system which can correct a fluctuation in an intensity distribution of illumination on an irradiated plane which follows a change of the form or the size of a secondary light source, and an exposure apparatus having such illumination optical system.
Also, as a result of further repeated experiments, or the like, it has been found that when at least a certain optical system (a movable lens, or the like) which constitutes the condenser optical system is moved in the direction of the optical axis in order to correct an unevenness in the intensity of illumination which is generated on an irradiated plane due to a change of the aperture form of an aperture stop or the like arranged on the exit side of the optical integrator, the optical characteristics such as the back focus or the telecentricity of the condenser optical system are changed to deteriorate the illumination condition, owing to the movement of the optical system.
Accordingly, another object of the present invention is, taking the above-mentioned problem into consideration, to provide an illumination optical system which can correct a change in optical characteristics generated due to the correction of the unevenness in the intensity of illumination following the change of the form or the size of the multiple light source by use of an adjusting mechanism of a simple structure, and an exposure apparatus provided with the illumination optical system.